Please let us know how your new unit works when you get it.
I'm sure that some of us are very interested in the outcome and whether or not that you will be satisfied with the 5 ton system on 3190 st ft, with 10 ft ceilings.
Thanks
Richard

07-18-2005, 06:34 PM

nathan9999

I'm either really confident in my calculations, or I'm starting to get worried.

I forgot to mention that my contractor actually was okay with a 4 ton solution. The HVAC-Calc program indicated a 4 ton when window coverings were included. And keep in mind I changed summer outside design temp to 110, about 6-7 degrees higher than program default.

Have tentatively decided on zoned Trane XL19i with Trane integrated zoning system. Wanted XL16i (R410A and scroll) but Trane zoning system does not support the XL16i.

No. The reason you hear that is because the full load is often between sizes and you most often dont go down.

07-18-2005, 10:21 PM

Panama

How did you specify your window glass in your Manual J calculation? Did you enter it as "tinted" or just "low e?" I ask because Manual J, 7th Edition does not handle modern windows very well.

Did you measure the window sizes by measuring the area that opens (per Manual J) or did you simply use the nominal sizes? This can make a difference of 15% or more in the heat gain from windows.

Two-foot eaves? Did you "shade" portions of the windows when you did the calculations? That can easily make a 10% or more difference in the load calculation. HVAC-Calc may not handle shading calculations, but the procedure is in Manual J itself.

I hope you have specified cellulose insulation, as it is far superior to fiberglass. There is a lot of evidence supporting this.

Kicking the design temperature up 6-7 degrees is not a good idea. It takes a long time for temperatures even 10 °F above design to raise the temperature inside a building more than a couple degrees. People and lights do a much better job of that. If an unusually hot day is predicted, just set the thermostat a little lower the night before.

The most comfortable home day in and day out is one with a slightly undersized system, as it runs continuously for hours on end. When it runs like that the air in the house mixes better and temperatures are more even throughout the house. Also, when the air is moving you feel cooler.

Did you use Manual S and Trane "Cooling Performance Data" to select your equipment? Most "shortcut" Manual J systems assume a 77% sensible heat ratio by having you add 30% to your sensible load; your house in Bakersfield probably will have a 90-95% sensible heat ratio. This makes selection of the blower, evaporator, and condesing unit very different.

I suspect you are going to be substantially oversized. If you're not, then there will be something wrong with the design, installation, or commissioning. It's criminal how many systems don't cool well simply because the are incorrectly charged.

07-19-2005, 07:41 AM

bornriding

Nathan

In my, very humble, opinion, manual 'j' sizes for energy efficiency, and not comfort.
At least in my area of the south, I have seen many (including my own home), that were sized for manual 'j' that after a few years would not keep up.
I bought my 1950 sq ft home with 2.5 ton HP when it was ten years old. Would not cool below 78 degrees if outdoor was 85 or over.
Manual 'j sizes for 93 OAT, 75 IAT, here. Ok if you like it 75 or above. But after a few years, when system not new anymore, and home is not as tight as it was when new, units tend to not be able to cool as easy.
I have been in business 15 years, and I size units for customer comfort, and I have 'never' had to go back to one because the customer wasn't happy with the system...never.
I've not had but one customer complain about electric bill, and he upgraded his system to larger unit, and wife kept thermostat on 70.(would not get there with old system) Have had several customers let us know that their electric bills had gone down, and were very happy with that.
Have replaced unit less than ten yrs old, due to not keeping temp during hot summer days. I assumed it was installed per manual 'j'.
So for comfort issues, I do not have faith in manual 'j'.

07-19-2005, 07:49 AM

Gunnery Sergeant Hartman

I don't know how anyone could want their temps much lower than 75?
I keep mine at 74 and we have to sit under blankets at night while watching tv.
My RH is 40% also...

07-19-2005, 09:05 AM

dash

Yes ,Man. J ,oversizes ,but it's not a lot,and it varies ,5 to 10% maximum,without user input error.
The Author states it so I'd say it's a fact.

07-19-2005, 09:14 AM

docholiday

Born,

With all due respect a unit should not lose capacity after a few years. If a homeowner fails to maintain their system, you cannot go back and blame the load calc.

If he designs his system for 107 out and 70 in, he'll need a 30 ton chiller.

07-19-2005, 09:58 AM

Panama

One problem with a 5-ton system in a dry climate is that you can't move enough air with a typical 5-ton blower to get good efficiency out of the system. It is costly to run a system at a 0.8 sensible heat ratio when all you need is 0.9.

It makes better sense to use a 4-ton condensing unit with a 5-ton blower and coil. Set the sucker on high speed and you will be cool, comfortable, and economical.

07-19-2005, 10:16 AM

ruud-man

Yellow Dot

Regarding your post about 74dF and the blankets. I'm curious why run the A/C at 74dF if it is too cool for comfort?

The reason I ask is I prefer a dry 78dF and my wife wants the temp lower, but then grabs a blanket or sweater. Me, I just watch dollars fly off to the utility company.

Best regards...

07-19-2005, 10:56 AM

nathan9999

Responses as best as I can.

Jacob, I think you're right. I am overstating the requirements by increasing the outside design temp.

I appreciate your comments on sizing. I'll discuss with my contractor. I have not used Manual S. I believe the contractor said he would use a "Ductalator" (can't be sure of spelling). I'll ask contractor about Trane Cooling Performance Data.

How would using a dual compressor system and VS alleviate oversizing issues, if at all?